"chuck" wrote in message
...
Paul wrote:
SNIP
As has been mentioned, using coax from the tuner to the end of the
backstay has two disadvantages: It adds shunt capacitance, and some
loss, which can makes the tuner's job more difficult. Also, it may arc
through from the center conductor to the shield, during operation at
certain frequencies where the voltage can rise to very large values.

SNIP
- Paul (wb6cxc)
- S/V VALIS -- PSC44 #16 -- Sausalito, California
- www.sailvalis.com



Hello Paul,

Part of the problem in analyzing a run of coax between the tuner and the
bottom of the backstay is agreeing on what the alternative is. If the
alternative is having the tuner right at the base of the backstay, then
that will usually give the best results. (But see below)

But if the tuner is, say eight (or more) feet away from the bottom of the
backstay and the run is more or less along the waterline, then that's not
going to be a very attractive alternative. On frequencies where that
length constitutes a current node, (especially at higher frequencies where
the antenna might be a short, automotive-type whip) that length is an
important part of the radiating system. But its radiation is essentially
into the water where it does no good.

A variation on that is where the connection between the tuner and the
"ground" is several feet long: a not unusual arrangement. In that case,
the antenna actually begins at the ground system and the ground wire
running to the tuner is a full, radiating part of the antenna!

So the rules are not just to avoid coax between the tuner and the
backstay, but to install the tuner as close to the backstay as possible
and to install the ground system as close as possible to the tuner.
Fortunately, these rules are tempered with the knowledge that many
installations work acceptably despite their departures from the ideal.
;-)

I would second Bruce's recommendations, and also point out that on the
Chesapeake and its tributaries as well as on the Great Lakes, you will
need to provide lots of area for your RF ground because of the water's
lack of salinity.

Chuck,

Completely agree. The conductors leaving the tuner are part of the
radiating antenna, so the orientation will make a difference, and using
shield-grounded coax will only make matters worse. The connection to the
ground also needs to be electrically short -- not just because it is a
radiator, but because the tuner may not function if it's ground is "hot",
and the coax feedline from the radio to the tuner will also be hot.

I was describing my installation because in my opinion it is an example of a
well-installed system. While I've seen some badly-designed boat antenna
systems, mine is hardly unique, and I think that some folks here are being
pessimistic about how well a good insulated-backstay antenna system can
operate. Boats are floating compromises. We do the best we reasonably can,
and that usually takes some education, but we need to recognize when we have
reached our own point of diminishing returns.

-Paul